A number of arthritic diseases are associated with chronic infection by obligate intracellular organisms. The goal of this proposal is to define the effects of two such pathogens on the ability of Class II histocompatibility molecules (MHC II) to present antigenic peptides. Thepathogens C oxiella burnetti and Chlamydia trachomatis were chosen for comparison because they are both implicated in the development of arthritic disease but reside in very different intracellular compartments. We can infect gamma-interferon treated HeLa cells with either organism to 80-90% efficiency, so will be able to use morphological and biochemical methods for analyzing the effects of infection on the MHC II peptide-binding pathway. We can assay by immunoprecipitation and immunoblotting the levels of HLA-DR associated with invariant chain and its functional fragments, as well as the level of HLA-DR associated with HLA-DM and with peptide. We will also measure the kinetics of cell surface expression of mature (antigen-occupied) MHC II during infection. Comparison of MHC II maturation in cells infected by Coxiella burnetti or with Chlamydia trachomatis will distinguish the pathogen-specific effects on antigen presentation and will identify critical stages in the MHC II presentation pathway that are targets for pathogen-mediated immune evasion. Furthermore, these studies will reveal how intracellular residence of pathogens could influence an immune response against other antigens presented by an infected cell. These studies will therefore elucidate mechanisms leading to chronic bacterial infection and to stimulation of autoimmunity, both of which are implicated in arthritic disease.